In order to write a character contrast, you need to follow the development of the characters throughout the play and ask yourself relevant questions about them.
In The Merchant of Venice these are some questions that will reveal Antonio's character: How does he view friendship? Why does he agree to Shylock's demands for Basanio's sake? How does he react when he learns that the debt -tantamount to his life- will be collected? What does he think of Shylock in particular and of the Jews in general? What is his private life like?
Several of the above questions can be asked about Basanio as well, although the answers are bound to be different. One important question to ask about both is what kind of love moves them.
You can think of others along the same lines. Once you have your answers, with close reference to the text, you will be ready to write your essay.